The result of a measurement on an animal is called a phenotype. Measurements should always be carried out very carefully and should be critically reviewed. Measurement errors determine how accurate phenotypes can be established.
Definition
A phenotype is the observed value of a trait. It is a consequence of all the genetic and environmental influences and interactions affecting the trait, including errors in measurements
These measurement errors might have a systematic and/or random character. Systematic errors might be caused by differences between animals for example: diet composition, age at measurement, training etc. Random errors, e.g. measurements errors, may cause a low repeatability for a trait. For instance, when you want to measure the length of an animal, and you repeat this ten times you will see a rather large variation in outcomes. Slight movements of the animal are the cause. When you measure pelvic height, you see a rather small variation in outcomes. Two measures of accuracy of measurement are relevant: repeatability and reproducibility, both are correlations between measures on the same animal.
Definition
The repeatability is defined as the extent to which measurements on the same object under similar conditions correspond with each other. It indicates how accurate a trait can be established. It is influenced only by measurement errors and time effects
When the repeatability is low for a trait, the heritability will also be low. That means that is difficult to improve that trait in a breeding program.
Definition
The reproducibility is defined here as the relationship between measurements in different locations and/or by different persons. Besides measurement errors and time effects it is also influenced by systematic effects e.g. of classifiers or technicians
When the repeatability for a phenotype is high and the reproducibility is low, than efforts need to be taken to standardize the measurement of the phenotype and to train classifiers or technicians. For instance body weight of the piglets is measured at weaning when the piglets are separated from the sow at a fixed time during the day and they are weighed at a fixed time of the day after a fixed time of feed restriction before transportation to the slaughter house. Then, it is possible to calculate daily gain during fattening without systematic errors. When scoring body traits of horses it is highly recommended to start with a training session for the inspectors and repeat such a training session at regular intervals. Otherwise, two judges might give (systematically) different scores for the same trait of the same animal, leading to a low reproducibility of the trait despite its high repeatability.